A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet (EUV) radiation which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.
EUV sources based on a Sn plasma do not only emit the desired in-band EUV radiation but also out-of-band radiation, most notably in the DUV range (100-400 nm). Moreover, in the case of Laser Produced Plasma (LPP) EUV sources, the radiation from the laser, usually at 10.6 μm, presents a significant amount of unwanted radiation. Since the optics of the EUV lithographic system generally have substantial reflectivity at these wavelengths, the unwanted radiation propagates into the lithography tool with significant power if no measures are taken.
In a lithographic apparatus, out-of-band radiation should be minimized for several reasons. Firstly, resist is sensitive to out-of-band wavelengths, and thus the image quality may be deteriorated. Secondly, unwanted radiation, especially the 10.6 μm radiation in LPP sources, leads to unwanted heating of the mask, wafer and optics. In order to bring unwanted radiation within specified limits, spectral purity filters (SPFs) are being developed.
Spectral purity filters can be either reflective or transmissive for EUV radiation. Implementation of a reflective SPF requires modification of an existing mirror or insertion of an additional reflective element. A transmissive SPF is typically placed between the collector and the illuminator and does not affect the radiation path, which is an advantage because it results in flexibility and compatibility with other SPFs.
Grid SPFs form a class of transmissive SPFs that may be used when the unwanted radiation has a much larger wavelength than the EUV radiation, for example in the case of 10.6 μm radiation in LPP sources. Grid SPFs contain apertures with a size of the order of the wavelength to be suppressed. The suppression mechanism may vary among different types of grid SPFs as described in the prior art and detailed embodiments further in this document. Since the wavelength of EUV radiation (13.5 nm) is much smaller than the size of the apertures (typically >3 μm), EUV radiation is transmitted through the apertures without substantial diffraction.
A further challenge with existing spectral purity filters is that they change the direction of the light from the EUV source. Therefore, if a spectral purity filter is removed from an EUV lithography apparatus, a replacement spectral purity filter should be added or a mirror at a proper angle should be introduced. The added mirror introduces unwanted losses into the system.
U.S. patent application publication no. 2006/0146413 discloses a spectral purity filter (SPF) comprising an array of apertures with diameters up to 20 μm. Depending on the size of the apertures compared to the radiation wavelength, the SPF may suppress unwanted radiation by different mechanisms. If the aperture size is smaller than approximately half of the wavelength, the SPF reflects virtually all radiation of this wavelength. If the aperture size is larger, but still of the order of the wavelength, the radiation is at least partially diffracted and may be absorbed in a waveguide inside the aperture.
Several prior art spectral purity filters (SPFs) rely on a grid with micron-sized apertures to suppress unwanted radiation. The approximate material parameters and specifications for these SPFs are known. However, a successful manufacturing method has not been described so far. Manufacturing is not straightforward at these specifications. The most challenging specifications are: apertures of typically 4 μm in diameter; a grid thickness of typically 5-10 μm; very thin (typically <1 μm) and parallel (non-tapered) walls between the apertures to ensure maximal EUV transmission.
U.S. Pat. No. 7,031,566 B2 discloses a filter, wherein the transmission spectrum is optimized by introducing at least one layer of substantially transparent dielectric material on the pore walls. The publication describes a method for the fabrication of the spectral filters, the method including: taking a semiconductor wafer having first and second surfaces wherein said first surface is substantially flat, producing a porous layer in the wafer starting from the first surface, coating the pore walls with at least one layer of transparent material, and subsequently removing the un-etched part of the wafer that remains under the porous layer. U.S. Pat. No. 7,031,566 B2 proposes to apply a transparent coating on the sidewalls of the pores for waveguiding of the desired wavelength.